Aluminum castings are manufactured in large numbers from a variety of aluminum alloys. Such castings find application where relatively low weight and high strength are desired. Among the families of aluminum casting alloys is the series of aluminum silicon alloys that also may contain small amounts of magnesium or magnesium and copper as alloying constituents.
It has long been recognized that the addition of silicon to aluminum dramatically increases the fluidity of such molten alloys for casting complicated shapes. Aluminum and silicon form an eutectic mixture at about 12 weight percent silicon, and hypoeutectic aluminum-silicon alloys, eutectic alloys and hypereutectic aluminum-silicon alloys are used to produce castings. For example, in sand mold castings a silicon content of about 5 to 7% by weight is often employed. For permanent mold castings, a suitable silicon content is often 7% to 9% by weight. For die casting, the silicon content may be about 8% to 12% by weight. There are hypereutectic aluminum-silicon casting alloys in which the silicon content is 16% to 20% by weight.
Magnesium or magnesium with small amounts of copper and nickel are used as additional strengthening elements in aluminum-silicon casting alloys. In suitably heat treated alloys these elements can form a variety of strengthening phases such as Mg.sub.2 Si, CuMgAl.sub.2 and CuAl.sub.2 in addition to the ubiquitous silicon. In addition, more complex intermetallic particles are formed containing various combinations of Ni, Cu, Mg, Si and Fe.
The metallurgical microstructure of aluminum-silicon castings normally comprise dendrites of aluminum-rich composition with silicon particles and precipitated alloying constituents distributed within the dendrite arms. Thus, the chemistry of aluminum-silicon alloys lends itself to hardening or strengthening by heat treatment. The purpose of such practices is to increase the hardness of the casting for improved machinability. An additional purpose is to increase its strength or produce mechanical properties associated with a particular material condition. Heat treatments are also used to stabilize mechanical and physical properties or to ensure dimensional stability as a function of time under service conditions.
There are a number of standard Aluminum Association heat treatment practices and designations for industry use. Among standardized temper designations applicable to casting are:
T4--in which the casting is solution heat treated, quenched and aged at ambient; PA1 T5--in which the casting is artificially aged by heating to a suitable temperature of the order of 200.degree. C. for a few hours; and PA1 T6--in which the casting is solution heat treated, quenched and then artificially aged at about 200.degree. C.
In the solution heat treatment, castings are heated to a temperature to dissolve the soluble constituents in the solid aluminum matrix. For a typical aluminum-silicon casting alloy a suitable solution heat treatment is in the range of about 500.degree. C. to 540.degree. C.
As stated the purpose of all such heat treatments is to increase the hardness and strength of aluminum castings or other work pieces at primarily room temperature. However, while aluminum alloys are not especially known as high temperature materials, some such alloys are used to make automotive pistons, for example, and other products that have utility at elevated temperatures. So it is very desirable to produce aluminum alloy castings with improved tensile strength at elevated temperatures, for example, of the order of 300.degree. C.